[Why]
drm_plane_cleanup does not free the plane.

[How]
Call drm_primary_helper_destroy which will also free the plane.
Signed-off-by: Harry Wentland <harry.wentland@amd.com>
Acked-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>